Allow indexing of &str in Rust

Allow indexing of &str in Rust

rust_slice_type_p was not recognizing &str as a slice type, so indexing
into (or making a slice of) a slice was not working.

2017-10-02  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_slice_type_p): Recognize &str as a slice type.

2017-10-02  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.exp: Test index of slice.